Several devices for detecting the angular position of a wheel driven by a timepiece motor have been proposed. Several documents concern the arrangement of optical devices comprising a light source and a light sensor, wherein the timepiece movement is arranged to vary the reception of light by the sensor in a controlled manner as a function of the angular position of the wheel concerned. Other documents propose the arrangement of capacitive sensors or inductive sensors. Some documents propose the arrangement of magnetized elements and at least one Hall sensor. These devices are all relatively expensive and complex, Further, they often result in a relatively large overall dimension and/or require specific machining of parts of the timepiece movement, notably of the plate of the wheel concerned.
To decrease the cost, complexity and overall dimensions of the device for detecting the angular position of a wheel, it has been proposed to introduce a “hard point” in one gear of the gear train comprising the wheel concerned, such a “hard point” consisting in adding an additional load or respectively a resistive torque for the motor driving the gear train limited to a restricted angular area of the wheel. Detection of this additional resistive torque by suitable detection means, notably by determining the torque required to make one motor step, makes it possible to detect the passage of a reference axis of the wheel concerned through a certain reference angle relative to the axis of rotation of the wheel.
A first device without an additional external sensor is disclosed in CH Patent 640098, which provides for the arrangement of a ferromagnetic element on the wheel plate in proximity to the toothing and a fixed magnet at the periphery of the wheel. During the rotation of the wheel, when the ferromagnetic element approaches the magnet, the magnet attracts it in the direction of rotation and thus the energy required to make one motor step decreases. However, once the angular position of the magnet has been passed, the magnet exerts a force in the direction opposite to rotation, which causes an increase in the energy required to make one step. A circuit detecting the energy of the electrical pulse provided by the motor with each step makes it possible to determine the step in which the ferromagnetic element was substantially facing the magnet. This system has various drawbacks. Firstly, it uses a magnet, which may affect other elements of the timepiece movement. Further, the magnetic force on the wheel may have an axial component that generates a torque on the wheel arbor and increases friction in the bearings. Next, the arrangement of the magnet at the periphery of the wheel requires a certain space to be freed inside the movement, which is not always easy. Finally, the magnet acts on the ferromagnetic element over a relatively large angular distance corresponding to several motor steps. Detecting the position of the reference axis of the wheel, defined by the ferromagnetic element, therefore requires analysing the behaviour of the motor over several steps. It is therefore proposed here to analyse the current curve for each pulse and to determine the evolution of certain specific parameters of this curve which are dependent on the torque provided to make the corresponding step.
A second device without an additional external sensor is disclosed in U.S. Pat. No. 6,414,908. This document teaches the arrangement of a “hard point” producing a localised high load for the stepping motor on one or more steps when the wheel is being driven. The detection of this load is achieved in a given example by measuring the length of the motor pulses. More precisely, it is arranged here that normal pulses are supplied with a first energy to achieve the stepping motion of the gear train. A detection device can determine whether the rotor has properly completed a step once a normal pulse has been supplied. If this is not the case, it is arranged in this embodiment that a first correction pulse is supplied with a second energy, higher than the first energy. In normal operation, without a hard point arrangement, the driving of the gear train is ensured by the normal pulses and the first correction pulses. However, the resistive torque generated by the hard point arrangement requires a second correction pulse with a third energy, higher than the second energy. Thus, any detection of non-rotation, once a first correction pulse has been supplied, is caused by the hard point, which thus makes it possible to determine the position of a reference wheel axis simply by determining the steps that required a second correction pulse. By way of example, if a substantially constant electrical power is supplied to the motor, the various pulses are distinguished by their different respective lengths.
U.S. Pat. No. 6,414,908 describes in detail the detection of the passage of a hand through the “12 o'clock position”, based on recording the steps that required a second correction pulse to be applied, but there is virtually no teaching as to the practical embodiment of a hard point. To produce an additional resistive torque, only two examples are briefly mentioned. The first variant proposes a local modification of the toothing profile. For the second variant, it is simply indicated that the additional resistive torque is generated by a cam. This second variant is vague and those skilled in the art are given virtually no practical teaching here. With regard to the first variant, it is not without interest, but no concrete example is given. It will be noted, however, that the implementation of this first variant poses certain technical problems. Firstly, making such a wheel with a non-uniform toothing complicates its manufacturing process. Next, given the manufacturing tolerances, it is not easy to ensure a hard point with a resistive torque whose value is within a given range. Finally, some gear play is generally necessary to ensure proper meshing. Creating a hard point by locally varying the toothing profile can easily result in impeding the rotation of the motor and thus the driving of the gear train associated with the motor.